A dermatoscope is a medical device used by dermatologists to image the skin. (Google Images) It's typically used as a first non-invasive step towards determining whether a skin lesion might be malignant.
Dermatoscopes have built-in illumination, and they almost always use 10x magnification.
Dermatoscopes can utilize either unpolarized or polarized light. More capable devices are able to switch between the two modes.
Polarized dermatoscopes can image slightly deeper, including the dermal-epidermal junction and the superficial dermis. Unpolarized dermatoscopes are preferred when the clinician needs to image the surface of the lesion.
"Polarised dermatoscopy uses cross-polarisation to view the structure. ... This technique minimises [surface] glare and allows better visualisation of the underlying structures".
Here's a good explanation[2] of how polarized light functions to provide deeper imaging. Notice the three paths that light can take: 1) surface glare, 2) superficial light, and 3) penetrating light.
Interface fluid increases the penetration of light. Interface fluids that are typically used are — 70% alcohol, liquid paraffin, water, and ultrasound gel.
Polarized dermatoscopes don't require interface fluid to get an acceptable brightness and quality. However, interface fluid does still improve image quality some.